The present invention relates to a process for separating monosilane from chlorosilanes-rich mixture. In particular, said process is downstream a process for continuously producing monosilane, the demand for which is recently increasing as a raw material for epitaxy of silicon with a high purity and for amorphous silicon for solar cells for example.
A known process for producing monosilane is a process for producing monosilane gas by disproportionating a hydrogenated silicon chloride such as trichlorosilane in the presence of a tertiary amine hydrochloride as a catalyst (JP-B-64-3804 and JP-B-63-33422).
Furthermore, another known process is a process for producing monosilane gas by packing a solid catalyst in a reaction column and disproportionating dichlorosilane therein (JP2,648,615). However, since the conversion reaction to monosilane is an equilibrium reaction, the equilibrated conversion ratio has not necessarily been high heretofore, from 10% to 18%, and a large-size apparatus has been required to achieve a desired production amount.
Another known process is a process for continuously producing monosilane readily and efficiently with a large production amount of monosilane from trichlorosilane and dichlorosilane as raw materials (production amount per hour in use of an apparatus with the same reaction performance). EP 2085358 discloses a process for continuously producing monosilane by means of a monosilane production apparatus comprising a reaction column, a plurality of upper condensers each of which has a reflux feed pipe serially connected to a top portion of the reaction column, a bottom reboiler of the reaction column, and an evaporation tank connected to a bottom portion of the reaction column; the process comprising supplying at least one of trichlorosilane and dichlorosilane to a middle stage of the reaction column, supplying at least one of a tertiary aliphatic hydrocarbon-substituted amine and a tertiary aliphatic hydrocarbon-substituted amine hydrochloride as a catalyst to an upper stage of the reaction column, introducing a resultant mixture containing monosilane, monochlorosilane, dichlorosilane, and trichlorosilane from the top portion of the reaction column to the plurality of upper condensers, separating monosilane from condensates containing monochlorosilane, dichlorosilane, and trichlorosilane at a temperature of from 50° C. to −50° C. in the upper condensers, recycling the condensates after separating monosilane, through the reflux feed pipes to the upper stage of the reaction column, bringing the condensates into contact with the catalyst in the reaction column, withdrawing a bottom recovery liquid containing tetrachlorosilane and the catalyst from the bottom portion of the reaction column, introducing the bottom recovery liquid into the evaporation tank, and recycling the catalyst recovered from the bottom portion of the evaporation tank, to the reaction column.
According to this document, the condensates at the temperature of from 50° C. to −50° C. are refluxed to the reaction column by means of upper condensers each of which has a reflux feed pipe serially connected to a top portion of the reaction column. The number of upper condensers with the reflux feed pipe is at least 2. A production amount (production amount per hour based on mole) of monosilane as the desired product depends on the number of condensers but use of too many condensers, for example, also decreases the production amount of monosilane. So it could be a main inconvenience in this kind of process.
Moreover, a temperature difference between the condensates of upper condensers adjacent to each other is appropriately determined in accordance with the number of upper condensers with the reflux feed pipe. Where a temperature of the condensate of the (i+1)th upper condenser from the top portion of the reaction column (i is an integer of at least 1) is Ti and a temperature of the condensate of the (i+1)th upper condenser is Ti+1, and when the number of upper condensers is from 2 to 5, the temperature difference is determined preferably in a range of Ti-Ti+1≧10° C. and more preferably in a range of from 15° C. to 100° C., depending on the specific number of upper condensers.
Furthermore, when the number of upper condensers is 3 or 4, the temperature difference is determined preferably in a range of Ti-Ti+1≧15° C. and more preferably in a range of from 20° C. to 60° C., depending on the specific number of upper condensers. If the temperature difference between the upper condensers is too small, a separation efficiency or yield of monosilane from the mixture containing monosilane, monochlorosilane, dichlorosilane and trichlorosilane might be decreased, or a recovery efficiency of monochlorosilane, dichlorosilane and trichlorosilane might be decreased. So it is understood that this process is very hard to be implemented and it is easy to make a mistake in the determination of the different parameters (number of condensers, temperature in each condenser . . . ).
Furthermore, it is necessary to implement a process with a reduction of refrigeration power compared to the process described above.
The present invention aims at solving the problem by removing the inconvenience of the process described above and to improve the reduction of refrigeration power.